Ukraine and Russia exchanged accusations over each others' actions around the Zaporizhzhia nuclear power plant on Friday as a team of inspectors from the United Nations nuclear watchdog tried to check the safety of the facility and avert a potential radiation disaster.

Ukraine's state nuclear company said the International Atomic Energy Agency (IAEA) mission had not been allowed to enter the plant's crisis center, where Ukraine says Russian troops are stationed and would struggle to make an impartial assessment.

Russian Defense Minister Sergei Shoigu said Ukraine was continuing to shell the plant, raising the risk of a nuclear catastrophe.

The site, 10 kilometers (6 miles) from Ukrainian positions across the Dnipro river, was captured by Russian forces soon after they invaded Ukraine in late February and has become the focus of concern.

It has come under repeated shelling over the past month, with Kyiv and Moscow trading blame for the firing. The plant is still run by Ukrainian staff and Russia has rejected calls for it to withdraw its troops.

IAEA chief Rafael Grossi and his team spent several hours at Europe's largest nuclear power plant on Thursday and intended to return on Friday across the frontlines to assess the damage.

Speaking after the initial visit, Grossi said the physical integrity of the plant had been violated several times and he was worried by the situation there.

Ukraine's state nuclear company Energoatom said it would be difficult for the IAEA team to make an impartial assessment due to Russian interference.

"The Russians did not allow the mission to enter the crisis center, where Russian military personnel is currently stationed, whom the IAEA representatives were not supposed to see," Energoatom said in a statement.

"The (Russian) occupiers lie, distort the facts and evidence that testify to their shelling of the power plant, as well as the consequences of damage to the infrastructure," it said.

Ukrainian President Volodymyr Zelenskyy said the mission could still have a role to play despite the difficulties met.

"Unfortunately, we haven't heard the main thing from the IAEA, which is the call for Russia to demilitarise the station," Zelenskyy said in a video streamed to a forum in Italy.

In Moscow, Defense Minister Shoigu rejected assertions by Kyiv and the West that Russia had deployed heavy weapons at the plant. He accused Ukraine of "nuclear terrorism" by shelling.

Shoigu repeated Moscow's insistence that Kyiv would be responsible for any escalation at the site.

He said Kyiv was "creating a real threat of nuclear catastrophe" and using Western-supplied weapons to attack the plant. He also accused the United States and European Union of "encouraging such reckless actions."

One of the plant's reactors was forced to shut down on Thursday due to shelling.

Several towns near the plant came under Russian shelling on Thursday, Zaporizhzhia regional council mayor Mykola Lukashuk said. Reuters was unable to confirm this independently.

Russia's ambassador to international institutions in Vienna said two IAEA inspectors would stay at the Zaporizhzhia plant on a permanent basis, the RIA Novosti news agency on Friday.

The IAEA's Grossi said on his return to Ukrainian-held territory on Thursday that his experts would stay at the facility.

He had been able to tour the entire site, seeing key areas such as the emergency systems and control rooms. His team would now need to finish its analysis of technical aspects.

"We are not going anywhere. The IAEA is now there, it is at the plant and it is not moving – it's going to stay there," Grossi told reporters once he had crossed back into Ukrainian-held territory.

Those experts, he said, would provide what he called an impartial, neutral, technically sound assessment of what was happening on the ground.

The plant sits on the south bank of a huge reservoir on the Dnipro River that divides Russian and Ukrainian forces in central southern Ukraine. Before the war, it supplied more than a fifth of Ukraine's electricity.

Counteroffensive

Ukraine started an offensive this week to recapture territory in southern Ukraine, mainly further down the Dnipro in neighboring Kherson province.

Both sides have claimed battlefield successes, although details have been scarce so far, with Ukrainian officials releasing little information.

Ukraine's southern command spokesperson, Natalia Humeniuk, said on Friday that Ukrainian troops had destroyed ammunition depots and pontoon bridges to hamper the movement of Russian reserves.

"Our successes are convincing and soon, we will be able to disclose more information," she said.

Moscow has denied reports of Ukrainian progress and said its troops had routed Ukrainian forces.

Reuters could not independently verify those claims.

Ukraine's general staff on Friday said Russian forces had shelled dozens of cities and towns, including Kharkiv - Ukraine's second largest city – in the north and Donetsk in the east.

More than seven million people have fled Ukraine, thousands have been killed and cities have been reduced to rubble in what Kyiv and the West call Russia's unprovoked war of aggression.

Moscow calls its actions a "special military operation" to rid Ukraine of nationalists and protect Russian-speaking communities.